The present invention relates generally to optical detection and, more particularly, to optical scanners used in the creation of color images.
Traditionally stepper motors that move in discreet steps have been used with linear image sensors in optical scanners such as are used, for example, in flatbed scanners, film scanners, and copiers, and certain digital cameras to capture images. Recently DC motors have replaced stepper motors in such devices in order to obtain increased positional accuracy in scanning, to increase scanning speed, and to reduce cost. Images are obtained via either reflected light as in the case of photographs and paper documents or transmitted light as in the case of transparencies, film negatives, and photographic slides.
One of the most popular image sensor technologies is that of charge-coupled devices (CCDs). Depending upon the application, accurate color reproduction by image sensors may pose special problems, as the intensity of the optical signal detected by color image sensors is dependent upon wavelength. Both the output intensity of light sources and the detection efficiency of detectors typically used in such systems are wavelength dependent. In addition, the transmission characteristics of transparent media is in general strongly wavelength dependent.
Images containing color are usually created by separating and detecting the image signal in three separate color bands which could be, for example, the red, green, and blue color bands. The use of different amplification gains for each of the three color signals detected could be used to compensate for the combined color dependency of the image creation system. However, as transparent media is strongly absorbing, especially in the blue color band, this solution may not produce an acceptable result due to noise in low signal level cases.
An alternative technique is to detect the optical signal in the green color band for longer than in the red color band and even longer in the blue color band. An acceptable result is obtainable for those cases wherein the scanning movement is stopped during the exposure, as for example when stepper motors are used. However, when movement of the image sensor occurs relative to the object during the scan, as for example with the use of DC motors, a rainbow effect is often observed caused by misalignment of the areas detected. In other words, the center of the area from which a blue signal is received is different from that from which a green signal is received and is different from that from which a red signal is received. In which case, an unacceptable image can result.
In representative embodiments, an optical scanning system is disclosed which comprises at least one first photosensitive detector that is sensitive to light in a first frequency band, an exposure control circuit that is capable of exposing the first photosensitive detector for a first time period and at least one second photosensitive detector that is sensitive to light in a second frequency band. The second photosensitive detector is fixed at a first location relative to the first photosensitive detector. The exposure control circuit is capable of exposing the second photosensitive detector for a second time period with the second time period being longer than the first time period and the midpoint of the first time period occurring at a time proximate to the time of the midpoint of the second time period.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.